In many data transmission methods, data are clustered into "packets," groups of bits that are transmitted in a burst. For example, a long message of one hundred thousand bits may be separated into one hundred packets of one thousand bits each. As each packet of the message is transmitted, a packet header is "attached" to the data of the packet, to serve as an "envelope" ensure the reliable delivery of the packet. The packet header includes a handful of bits that tell the receiver that a message is in progress and allows the receiver to synchronize to the transmitter. Optionally, the next few bits of the packet header identify the receiver (for instance, in a cellular phone system, many telephones in the area receive each packet broadcast from the transmitter, but each telephone only "pays attention" to those packets addressed to it), and possibly the sender (in the manner of a "return address" on a paper envelope), and other information to assist in packet delivery. The packet header is followed by the data bits of the packet (a thousand bits, in the example), conveying part of the actual message that the transmitter wants to send. After the packet is complete, the transmitter may wait a short time to allow some other transmitter a chance to transmit a packet. Then the transmitter sends the next packet, with its own packet header and packet body.
Referring to FIG. 1, it is known that the bits of a packet can be transmitted by modulating the phase of a transmission signal wave. For instance, the five bit message "11010" might be encoded by the wave form 100. The signal is transmitted as a wave of wavelength .lambda.. The transmitted wave is divided into bit times of length b. (For ease of illustration, each bit in FIG. 1 is encoded in two wave periods, with sharp transitions at the boundaries between bit times b. In more typical applications, a single bit is encoded as several, tens, hundreds, or thousands of wave periods, with a more gradual transition between the wave forms representing the successive bits.) Each of the three One bits are transmitted by a wave with phase .phi.=.pi./4 (note that each of the One bits begins with the wave going down, at points 110, 112 and 114), and the Zero bits are transmitted by a wave with phase .phi.=5.pi.4 (note that the Zero bits begin with the wave going up at points 116 and 118).
Referring to FIGS. 2a and 2b, a signal transmitted from a transmitter often arrives at the receiver in a distorted condition. FIG. 2a shows a wave as transmitted (the heavy line 200 indicates the envelope surrounding the higher frequency carrier wave 202). Note that the peaks 210 and zero crossings 212 occur at uniformly spaced time intervals. The wave arrives at the receiver, however, in a distorted condition as shown in FIG. 2b. For many transmission channels, for instance the radio transmissions used in cordless or cellular phones, the signal may travel from transmitter to receiver by several paths simultaneously, reflecting from walls, buildings, etc. Some paths take longer than others, so that the signal arriving by one path interferes with the signal arriving by another. In one familiar example, this "multipath distortion" may be visible as a television "ghost." In the distorted received signal 250, the peaks 260 and zero-crossings 262 of the received signal are not uniformly spaced as in the original, which makes it difficult for the receiver to recover the transmitted signal.
In digital transmission where each bit of the transmission is preceded and followed by other bits, multipath distortion causes the successive bits to smear into each other. A given reflection of a given bit may arrive at the transmitter simultaneously with other reflections of several other bits. This smearing is called "inter-symbol interference." Inter-symbol interference produces multiple ghosts, each time- and phase-shifted relative to the main signal. If the effects of multipath inter-symbol interference are not removed, the message may be decoded incorrectly.
A particular difficulty arises at the beginning of each packet, especially in transmission of digital data, whether the transmitted data are digitally-encoded voice communications, FAXes, or computer data. Because the crystal oscillators of the transmitter and receiver inevitably oscillate at slightly different frequencies, it is difficult for the receiver to identify the instant at which a transmission signal begins, and difficult to establish a synchronized agreement as to the time at which each subsequent bit begins. It is therefore difficult to identify the wave phase for given bits of the message. Because either the transmitter or receiver may be mobile, both the amplitude of the signal at the receiver and the multipath distortion characteristics of the transmission channel may vary from moment to moment, further complicating the recovery of the digital data from the received signal.
Referring to FIG. 3, one known packet header 300 includes several portions that consecutively allow the receiver to determine various characteristics of the received signal wave. Once all of these characteristics are determined, the receiver can "lock on" to the signal and receive actual data bits of the packet reliably. During a first portion 310 of the packet header, the amplitude of the wave gradually "ramps up," and the receiver detects that a transmission is in progress. During a second portion 312, the receiver recovers the frequency of the carrier wave. During second portion 312, circuitry in the receiver is tuned to match the receiver's carrier frequency to that of the transmitter (so that the transmitter and receiver precisely agree on the wavelength .lambda. of FIG. 1). This is called "carrier recovery." During a third portion 314, the automatic gain control (AGC) of the receiver operates to compensate for varying signal strength (for instance, if the receiver is moving away from the transmitter, the receiver amplification gain is increased to compensate for the gradual decrease in signal strength). During a fourth portion 318, during receipt of a "synch word," the receiver chooses a point to sample the wave for each bit, and establishes a sampling clock at which future bits of the packet will be sampled. During a fifth portion 320, equalizer training data are received, and the receiver calibrates or "trains" the equalizer (another component of the receiver that filters out distortion in the received signal) to recognize and remove the inter-symbol interference from the received signal. It is only after all five of these overhead portions 310-320 of the packet header have been transmitted, and the receiver has calibrated itself to the signal, that the actual data 322 of the packet can be properly received.
Both synch word 318 and equalizer training data 320 are sequences of bits pre-agreed between the transmitter and receiver, and are typically the same for each packet transmitted. For instance, known systems use a Barker or PN sequence. Because of the inter-symbol interference described above in connection with FIG. 2b, each bit interval b will be somewhat smeared with parts of the signal from previous or following bits. During the synch word, the receiver compares the signal it actually receives to the signal that it knows was transmitted to rate out the multipath echoes from the strongest signal, and then chooses a time relative to the beginning of each interval b at which the signal for the respective bit is at its strongest, relative to the inter-symbol interference introduced from previous or following bits. Specifically, the receiver convolves the received signal with a constructed ideal signal to extract the impulse response of the channel; the maximum of is impulse response marks the preferred sampling time for each bit.
It is known to transmit multiple copies of the synch word bits. If the agreed synch word is, for instance, the bits "11010" , the transmitter and receiver might pre-agree that the synch word is to be transmitted in triplicate, e.g., "111 111 000 111 000, " so that the receiver can more accurately select a preferred sampling point.